It is often necessary in the motion picture and television broadcasting industries to synchronize an audio recorder to a video device, such as a camera or video recorder, particularly when the audio portion of a scene is recorded with equipment separate or remote from the video recording equipment. For example, film recording in the field utilizes cameras which are generally heavy and cumbersome and thus not easily carried to film scenes at rapidly changing locales. Thus, a portable audio tape device, remote and separate from the camera, is usually provided to record the audio portion of the scene. The audio tape usually includes a track which has a recorded pilot tone, either a nominal 60 Hertz sine wave or a modulated signal, which will allow the audio tape speed to be frequency synchronized to a master signal, and thus to the video tape or film, at the broadcast or film studio. The actual device which accomplishes this is known as a resolver. In essence, both the output signal from a master sync generator, typically 60 Hertz (generated at the studio) and the signal from the tape pilot track are coupled to the resolver, the resolver generating a signal proportional to the difference in frequency between the two signals. The difference signal is coupled to a motor via a voltage controlled oscillator (VCO) to drive the audio tape capstan, the capstan being increased or decreased in speed depending upon the variation of the output frequency of the VCO from a nominal value, typically 9600 Hertz. The speed of the tape is thus adjusted until it reaches the speed demanded by the master synchronization signal for frequency synchronization.
In addition to bringing the audio tape into frequency (speed) synchronization with the master signal, it is also necessary to phase synchronize the two signals. Many prior art systems combine the frequency/phase synchronization adjustment in one circuit. However, doing so often is not appropriate since there are trade-offs between phase and frequency adjustments when a single circuit is utilized to accomplish both functions. In particular, a relatively long time lag usually results between a validated input and the validated output at the low system frequencies utilized. Further, the prior art resolvers are generally configured to be responsive only to very slight variations (typically .+-.10%) from the 60 Hertz frequency established by the master generator. It may be necessary, however, for the resolver to be responsive to greater frequency variations. For example, if the audio device batteries are weak when the audio recording is made so that the tape is driven at a speed corresponding to a 40 Hertz pilot signal, instead of the 60 Hertz nominal value (a variation of approximately 33%), prior art resolvers would be unable to respond to bring the audio tape to proper speed (The same would hold true if it was necessary to vary the speed of the master signal significantly from the nominal value to increase or decrease the playing time of the film or videotape). In addition, the phase lock capabilities of these resolvers tend to vary with the speed demanded by the master signal.
As is well known, audio tape can be damaged, or separate tape portions spliced together. Prior art resolvers will detect the splice in repaired or edited tape, treat it as a pilot phase error, and speed up the tape when locking thereto. This can cause noticeable transport wow or flutter. An additional disadvantage is that after frequency and phase synchronization has been accomplished and the tape subsequently has been stopped, the resolver has to repeat such synchronization after the tape has restarted, thus slowing overall system operation.
A frequency-to-voltage converter is required in most prior art resolver systems. Frequency-to-voltage conversion is a fairly common operation in electronic circuitry; the typical method involves converting an input frequency into a pulse train and then integrating the pulses to provide an average DC voltage output proportional to the input frequency. One major trade-off with this method is conversion response time vs. allowable ripple on the output voltage: at low input frequencies, very slow integration time constants must be utilized to minimize output ripple, degrading response time; conversely, rapid response time dictates a rapid integration of the frequency pulses, creating a large ripple component on the output signal at low input frequencies.
It is possible to multiply the input frequency before conversion to reduce output ripple without adversely affecting response time; simple frequency doubling can be effected if the input signal maintains a constant 50% duty cycle at all frequencies. More complex multiplication schemes involve response lags and conversion uncertainties of their own which can compound the response/ripple tradeoff. All of these methods will only reduce the output ripple, and at low frequencies the response/ripple tradeoff must still be addressed.
Digital approaches may be employed to eliminate periodic ripple on the output voltage; a typical method would be to time the interval between repetitions of the input signal and convert the relative number obtained into a voltage, which would be proportional to the input frequency. In many applications, however, the need for a stable timing source, counter, latches, and a digital-to-analog converter create a situation untenable from both cost and component count considerations.